Magnesium alloy and casting

ABSTRACT

A magnesium alloy containing aluminum, manganese and calcium includes 6˜12% by weight of aluminum, 0.1˜1.5% by weight of manganese, a calcium/aluminum mass ratio being 0.55˜1.0, and balance being magnesium and inevitable impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C §119with respect to Japanese Patent Application 2006-019632, filed on Jan.27, 2006, the entire content of which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates to a magnesium alloy and a casting, each of whichexcels in heat resistance and castability.

BACKGROUND

In the industrial world such as automobile industry, magnesium alloyshave been applied for the purpose of weight reduction and a scope of theapplication is expected to expand in the future. Particularly, theapplication to peripheral components of engines and the likes, whichhave effects in weight reduction, is considered. However, high heatresistance is required for the peripheral components of engines and thelikes, and thus there is a need for developments for magnesium alloyswhich excel in heat resistance. Magnesium-aluminum-silicon based alloys,magnesium-aluminum-RE based alloys, and the likes have been developedfor enhancement of the heat resistance of conventionalmagnesium-aluminum based alloys. However, these alloys are notsufficient in terms of corrosion resistance, castability and cost.Magnesium-aluminum-calcium based alloys, which are excellent in hearresistance, corrosion resistance, and castability compared to theaforementioned alloys, have been developed. For example, an aspect thata magnesium-aluminum-calcium based alloy has high strength and isexcellent in the castability is disclosed in JP8-269609A. Enhancement ofstrength of a magnesium-aluminum-calcium based alloy by addition ofstrontium is disclosed in JP 2001-316752A. Enhancements of strength byincreasing the amounts of aluminum and calcium compared to knowninventions are disclosed in JP2004-23867A and JP2005-113260A.

More specifically, a magnesium-aluminum-calcium alloy, which contains1.0˜5.0% aluminum, 0.3˜3.0% calcium, is disclosed in JP8-269609A. InJP2001-316752A, a magnesium alloy for die-casting is disclosed and themagnesium alloy contains 2.0˜6.0% aluminum and 0.3˜2.0% calcium, and0.01˜1.0% strontium. In JP2004-238676A, a magnesium alloy, whichcontains 4.7˜7.3% aluminum, 1.8˜3.2% calcium, 0.0˜0.8% zinc, 0.3˜2.2%tin, is disclosed. In JP2005-113260A, a magnesium alloy is disclosed.The magnesium alloy contains more than 6%˜10% aluminum, 1.8˜5% calcium,0.05˜1.0% strontium, and 0.1˜0.6% manganese and the calcium/magnesiummass ratio is set to 0.3˜0.5%.

The industrial world requires to use magnesium alloys in severeconditions such as in higher heat and under larger loading stress. Heatresistance of the magnesium-aluminum-calcium based alloys which havebeen proposed so far is not necessarily adequate under the severecircumstances. Therefore, further improvements in the heat resistanceare required.

The present invention has been made in view of the above circumstances,and provides a magnesium alloy which excels in heat resistance andcastability for further improvement of the heat resistance.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a magnesium alloycontaining aluminum, manganese and calcium includes 6˜12% by weight ofaluminum, 0.1˜1.5% by weight of manganese, a calcium/aluminum mass ratiobeing 0.55˜1.0, and balance being magnesium and inevitable impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of the presentinvention will become more apparent from the following detaileddescription considered with reference to the accompanying drawings,wherein:

FIG. 1 is a graph illustrating a relationship between an amount ofaluminum and an amount of calcium and a Ca/Al mass ratio;

FIG. 2 is a conceptual scheme diagrammatically illustrating a cuttingform of a ring test piece for measuring a bolt-loading of a bolt;

FIG. 3 is a conceptual structure view schematically illustrating ameasurement of the bolt-loading of the tightened bolt;

FIG. 4 is a perspective view illustrating a test piece for evaluatingcasting cracks;

FIG. 5 is a graph illustrating results of minimum strain rates;

FIG. 6 is a photomap of a metal structure;

FIG. 7 is a photomap of a metal structure;

FIG. 8 is a photomap of a metal structure;

FIG. 9 is a photomap of a metal structure;

FIG. 10 is a photomap of a metal structure;

FIG. 11 is a photomap of a metal structure;

FIG. 12 is a photomap of a metal structure; and

FIG. 13 is a photomap of a metal structure.

DETAILED DESCRIPTION

A magnesium alloy, according to examples of the embodiments 1 and 2 ofthe invention, contains aluminum, calcium, and manganese. The magnesiumalloy contains 6˜12% by weight of aluminum and 0.1˜1.5% by weight ofmanganese, and calcium/magnesium mass ratio is in a range of 0.55˜1.0.The balance is composed of magnesium and inevitable impurities.Therefore, the magnesium alloy, according to the examples of theembodiments 1 and 2 of the invention, is amagnesium-aluminum-calcium-manganese based alloy. Reasons for definitionof composition will be explained below. In the specification, unlessotherwise indicated, % regarding contents or amounts is % by weight.

(6˜12% Aluminum)

Aluminum contributes to improvement of castability, especially improvesfluidity of the alloy. Aluminum also contributes to enhancement of alloystrength to improve mechanical properties. However, an excessive amountof aluminum tends to lower the ductility and the strength. In the caseof an insufficient amount of aluminum, it is difficult to obtain theadequate heat resistance due to shortage in an absolute amount of anAl₂Ca (Mg) phase. Further, temperature of a liquid phase of the alloybecomes high and a liquidus-solidus temperature range is broadened,resulting in casting cracks. When the amount of aluminum exceeds 12%,the rough and large Al₂Ca (Mg) phase as a primary crystal is easilycrystallized and the castability is considerably lowered. Consideringthese circumstances, 6˜12% of aluminum is contained in the alloy.

In this case, the amount of aluminum may be set to equal to or greaterthan 6% and may exceed 6%. Thus, the amount of aluminum may be set to6˜10%, 6.1˜9%, 6.2˜8.5%, and so on. A lower limit of aluminum may be setto 6.05%, 6.1%, 6.2%, 6.4%, and 6.6% as an example. An upper limit ofaluminum, which may be paired with the lower limit described above, maybe set to 11.5%, 10.5%, and 9.5% as an example. However, the lower andupper limits are not limited to these figures. In this specification,the word “equal to or less than” is meant to describe the valueincluding the stated value and the words “exceed” and “less than” aremeant to describe the value not including the stated value.

(Calcium/Aluminum Mass Ratio is 0.55˜1.0)

Calcium/aluminum mass ratio affects formation of a β phase (Mg₁₇Al₁₂). Amelting point of the β phase (Mg₁₇Al₁₂) is relatively low and is likelyto be formed in a grain boundary. If the formation of the β phase islarge, boundary sliding is likely to occur in a high temperature rangeand it is difficult to attain the satisfactory heat resistance. When theabove-mentioned mass ratio is less than 0.55, the β phase is likely toappear. Consequently, the heat resistance is lowered. On the other hand,when the calcium/aluminum mass ratio exceeds 1.0, an Mg₂Ca (Al) phaseshows a relative increase to considerably lower the castability. If thecalcium/aluminum mass ratio is in a range of 0.55˜1.0, it is possible torestrain the formation of the β phase in the structure. Thus, the βphase is not or unlikely to be formed. It is preferable to restrain theβ phase in terms of the area ratio in a microscope field. Thus, the arearatio of the β phase should be equal to or less than 0.5%, and it isfurther preferable that the area ratio is equal to or less than 0.2% or0.1%. Alternatively, it is preferable that the β phase does not existvirtually. Therefore, it is more preferable that the area ratio is 0.0%.In regard to the area ratio, % is not weight %.

Considering the above circumstances, it is preferable to set thecalcium/aluminum mass ratio to 0.58˜0.90 or 0.6˜0.88. A lower limit ofthe calcium/aluminum mass ratio may be set to 0.58, 0.60, 0.62, 0.65 orthe likes as an example and an upper limit may be set to 0.98, 0.95,0.90, 0.88 or the likes as an example to be paired with each lower limitdescribed above. However, the upper and lower limits are not limited tothese figures.

When the calcium/aluminum mass ratio is 0.55˜1.0, a minimum value ofcalcium is 3.3% (6%×0.55=3.3%), andamaximumvalue of calciumis 12%(12%×1.0=12%). Thus, the amount of calcium is set to 3.3˜12%, and theamount of calcium may be set to 4˜11%, 5˜10%, or 6˜9% as an example.However, the amount of calcium is not limited to these figures.

FIG. 1 illustrates a relationship between the amount of aluminum and theamount of calcium in the magnesium alloy. As illustrated in FIG. 1, acharacteristic line K1 represents a case that the Ca/Al mass ratio=1.00(atomicity ratio: 1/1.49). A characteristic line K2 represents a casethat the Ca/Al mass ratio=0.550 (atomicity ratio: 1/2.7). Acharacteristic line K3 represents a case that the Ca/Al mass ratio=0.500(atomicity ratio: 1/2.98). A characteristic line K4 represents a casethat the Ca/Al mass ratio=0.300 (atomicity ratio: 1/4.95).

In FIG. 1, ♦ mark represents alloys whose bolt-load retention rates areequal to or greater than 90, provided that the bolt-load retention rateof ADC 12 is 100 at 175 degrees C. ◯ represents alloys whose bolt-loadretention rates are equal to or greater than 80, provided that thebolt-load retention rate of ADC12 is 100 at 175 degrees C. In FIG. 1, anarea KA represents a range of the calcium and aluminum amounts specifiedin the embodiments 1 and 2 of the invention. An area KB represents arange specified in JP2005-113260A. The formation of the β phase isrestrained in the area KA. Considering the Ca/Al ratio, the β phase ismore likely to be formed in the area KB.

(0.1˜1.5% Manganese)

Manganese contributes to improvement of corrosion resistance and aninsufficient amount of manganese lowers the corrosion resistance. Also,an excessive amount of manganese is not completely melted in a moltenmetal, and thus it is not possible to attain adequate effects for thecorrosion resistance and the heat resistance. Considering thesecircumstances, the amount of manganese is set to 0.1˜1.5%. For thereasons stated above, for example, the amount of manganese may be set to0.12˜1.3%, 0.2˜1.0%, and 0.3˜0.8%. A lower limit of manganese may be setto 0.15%, 0.20%, or 0.30% as an example and an upper limit of manganesemay be set to 1.3%, 1.2%, 1.0%, or 0.8% as an example to be paired withthe lower limit described above. However, the upper and lower limits arenot limited to these figures.

In the magnesium alloy, according to the examples of the embodiments ofthe invention, preferably, it is possible to contain at least one ofequal to or less than 1.5% of strontium, equal to or less than 2.5% ofrare earth elements, equal to or less than 1% of silicon, and equal toor less than 2% of tin. A phase, which is formed when at least one ofstrontium, rare earth elements, silicon, and tin are added, is differentfrom the Al₂Ca (Mg) phase, however, has a similar effect with the Al₂Ca(Mg) phase for the heat resistance, and contributes to furtherimprovement of the heat resistance. Moreover, additions of strontiun andrare earth elements improve the corrosion resistance of castings.Silicon and tin are effective for improvement of the castability.However, according to the magnesium alloy of the embodiments of theinvention, strontium, rare earth elements, silicon and tin do not haveto be contained unless there are particular needs.

(Equal to or Less Than 1.5% Strontium)

Strontium has an advantage in the improvement of the heat resistance.However, if strontium is contained more than the amount stated above,Mg—Al—Sr based chemical compounds or formation of Al₄Sr are/is increasedand the ductility is lowered. Considering the circumstances, whenstrontium is contained, the amount of strontium should be equal to orless than 1.5%. In this case, the amount of strontium may be less than1.3% or less than 1.1%. A lower limit of strontium may be set to 0.1%,0.2% or 0.3% as an example and an upper limit of strontium may be set to1.4% or 1.3% as an example to be paired with the lower limit. However,the lower and upper limits are not limited to these figures.

(Equal to or Less Than 2.5% Rare Earth Elements)

Rare earth elements contribute to the improvement of the heat resistanceby solid solution strengthening by being solved in a primary crystal ofa-magnesium matrix. The rare earth elements form compound phases in thegrain boundary of the primary crystal of the α-magnesium matrix torestrain boundary sliding to contribute to the improvement of the heatresistance. However, an excessive amount of rare earth elements tends tolower the ductility, the strength, the fluidity and the corrosionresistance. Considering the circumstances, if rare earth elements arecontained, the amount of rare earth elements should be equal to or lessthan 2.5%. In this case, the amount of rare earth elements may be equalto or less than 2.3% or equal to or less than 2.0%. A lower limit of therare earth elements may be set to 0.1%, 0.2%, 0.4% or 0.6% as an exampleand an upper limit of the rare earth elements may be set to 2.4% or 2.3%to be paired with the lower limit described above. However, the upperand lower limits are not limited to these figures.

It is costly to separate the rare earth elements as elementarysubstance, thus misch metal is employed as the rare earth elements.Generally, misch metal is a rare earth elements alloy mainly includingat least one of cerium, lanthanum, praseodymium and neodymium. Any oneof cerium series misch metal, neodymium series misch metal, andlanthanum series misch metal can be used. Elementary substance ofcerium, lanthanum, praseodymium, neodymium, or the likes may be useddepending on conditions. Further, other rare earth elements may be used.When the amount of calcium is relatively large, the castability may belowered. Thus, the amount of the rare earth elements may be reduced to0˜2%. However, the amount of the rare earth elements is not limited tothose values.

(Equal to or Less Than 1% Silicon)

Silicon is effective for the improvement of the heat resistance and thecastability. However, if an amount of silicon is excessive, the amountof crystallization of Mg₂Si compounds is increased to lower theductility and the strength. Therefore, if silicon is contained, theamount of the silicon should be equal to or less than 1%. Particularly,it is preferable that the amount of silicon is equal to or less than0.8% or equal to or less than 0.6%. However, the amount of silicon isnot limited to these figures.

(Equal to or Less Than 2% Tin)

Tin contributes to the improvement of the heat resistance by beingsolved in the primary crystal of the α-magnesium matrix. Furthermore,tin contributes to the improvement of the castability by crystallizingin the grain boundary and spaces between dendrite cells at nearly an endof complete solidification. However, an excessive amount of tin has adisadvantage for weight saving as tin has large specific gravityapproximately 7.3. For the reason, when tin is contained, the amount oftin should be equal to or less than 2%. In this case, the amount of tinmay be set to 0.1˜1.8%, 0.1˜1.0%, and 0.2˜0.8%, and so on. Asconsidering above circumstances, a lower limit of tin may be set to0.15%, 0.2% or 0.3% as an example and an upper limit of tin may be setto 1.8% or 1.5% as an example to be paired with the lower limitdescribed above. However, the upper and the lower limits are not limitedto these figures.

(Castings)

Magnesium alloys, according to the examples of the embodiments of theinvention, have the good castability and are suitable for die casting,gravity die casting, sand casting and the likes. Cold chamber system orhot chamber system can be used for die casting. The magnesium alloysaccording to the example of the embodiments of the invention isapplicable to components which require both the weight saving and theheat resistance. For example, the magnesium alloy is applicable for acylinder head cover, a cylinder block, a piston, and a transmission caseof vehicles. However, the application is not limited to thesecomponents.

Hereinafter, the embodiments of the invention are specificallydescribed.

As a series of the examples of the embodiment 1, ingredients areformulated so that manganese is fixed at 0.3% and the amount ofaluminum, the amount of calcium, the Ca/Al ratio is able to be changed.Similarly, as a series of the examples of the embodiment 2, ingredientsare formulated so that the amounts of aluminum, calcium, and manganeseare changed based on the compositions shown in Table 2, and the Ca/Alratio is also changed. In the series of the examples and comparativeexamples of the embodiment 2, the amount of strontium, the amount ofmisch metal, the amount of silicon, and the amount of tin may becontained.

Ingredients are melted at a gas melting furnace by a flux free method.Then, temperature of the molten metal is held at 690 degrees C. and themolten metal is charged into a molding cavity of a die casting mold of7.8 MN die casting machine to cast test pieces (die castings).Meanwhile, the compositions shown in Tables 1 and 2 are desired values.

TABLE 1 (Embodiment 1) Strength Properties Bolt-load retention rate(ratio of the Composition test piece to Castabiity Al Ca Mn Ca/Al βphase ratio ADC12) Casting Overall [mass %] [mass %] [mass %] [—] [%][%] Evaluation cracks evaluation Example 1-1 6.00 3.30 0.30 0.55 0 81 ∘Not observed ∘ Example 1-2 6.00 3.60 0.30 0.60 0 82 ∘ Not observed ∘Example 1-3 6.00 4.50 0.30 0.75 0 83 ∘ Not observed ∘ Example 1-4 6.005.28 0.30 0.88 0 84 ∘ Not observed ∘ Example 1-5 6.00 6.00 0.30 1.00 084 ∘ Not observed ∘ Example 1-6 7.00 3.85 0.30 0.55 0 83 ∘ Not observed∘ Example 1-7 7.00 4.20 0.30 0.60 0 86 ∘ Not observed ∘ Example 1-8 7.005.00 0.30 0.71 0 85 ∘ Not observed ∘ Example 1-9 7.00 6.16 0.30 0.88 087 ∘ Not observed ∘ Example 1-10 7.00 7.00 0.30 1.00 0 87 ∘ Not observed∘ Example 1-11 9.00 4.95 0.30 0.55 0 81 ∘ Not observed ∘ Example 1-129.00 5.40 0.30 0.60 0 84 ∘ Not observed ∘ Example 1-13 9.00 6.50 0.300.72 0 85 ∘ Not observed ∘ Example 1-14 9.00 7.92 0.30 0.88 0 85 ∘ Notobserved ∘ Example 1-15 9.00 9.00 0.30 1.00 0 85 ∘ Not observed ∘Example 1-16 12.00 6.60 0.30 0.55 0 82 ∘ Not observed ∘ Example 1-1712.00 7.20 0.30 0.60 0 83 ∘ Not observed ∘ Example 1-18 12.00 9.00 0.300.75 0 89 ∘ Not observed ∘ Example 1-19 12.00 10.58 0.30 0.88 0 88 ∘ Notobserved ∘ Example 1-20 12.00 12.00 0.30 1.00 0 85 ∘ Not observed ∘Comparative 4.00 3.00 0.30 0.75 0 77 x Not observed x example 1-1Comparative 7.00 1.00 0.30 0.14 4 47 x Not observed x example 1-2Comparative 7.00 3.00 0.30 0.43 0.7 68 x Not observed x example 1-3Comparative 9.00 3.00 0.30 0.33 3 55 x Not observed x example 1-4Comparative 9.00 4.00 0.30 0.44 0.9 69 x Not observed x example 1-5Comparative 12.00 5.00 0.30 0.42 1.5 61 x Not observed x example 1-6Comparative 13.00 10.00 0.30 0.77 0 82 ∘ Observed x example 1-7Comparative 7.00 9.00 0.30 1.29 0 88 ∘ Observed x example 1-8

TABLE 2 (Embodiment 2) Strength Bolt-load retention rate (ratio of testComposition piece to ADC Al Ca Ca/Al Sr Mm Si Sn Mn 12) Overall [mass %][mass %] [—] [mass %] [mass %] [mass %] [mass %] [mass %] [%] evaluationExample 2-1 6.00 3.30 0.55 0.50 0.00 0.00 0.00 0.30 83 ∘ Example 2-26.00 6.00 1.00 0.50 0.00 0.00 0.00 0.30 86 ∘ Example 2-3 7.00 4.20 0.600.50 0.00 0.00 0.00 0.30 88 ∘ Example 2-4 9.00 6.50 0.72 0.50 0.00 0.000.00 0.30 87 ∘ Example 2-5 12.00 6.60 0.55 0.50 0.00 0.00 0.00 0.30 83 ∘Example 2-6 12.00 12.00 1.00 0.50 0.00 0.00 0.00 0.30 87 ∘ Example 2-76.0 3.30 0.55 1.50 0.00 0.00 0.00 0.30 87 ∘ Example 2-8 6.0 6.00 1.001.50 0.00 0.00 0.00 0.30 90 ∘ Example 2-9 7.00 4.20 0.60 1.50 0.00 0.000.00 0.30 92 ∘ Example 2-10 9.00 6.50 0.72 1.50 0.00 0.00 0.00 0.30 91 ∘Example 2-11 12.00 6.60 0.55 1.50 0.00 0.00 0.00 0.30 82 ∘ Example 2-1212.00 12.00 1.00 1.50 0.00 0.00 0.00 0.30 91 ∘ Example 2-13 6.00 3.300.55 0.00 2.50 0.00 0.00 0.30 83 ∘ Example 2-14 6.00 6.00 1.00 0.00 2.500.00 0.00 0.30 86 ∘ Example 2-15 7.00 4.20 0.60 0.00 2.50 0.00 0.00 0.3088 ∘ Example 2-16 9.00 6.50 0.72 0.00 2.50 0.00 0.00 0.30 87 ∘ Example2-17 12.00 6.60 0.55 0.00 2.50 0.00 0.00 0.30 83 ∘ Example 2-18 12.0012.00 1.00 0.00 2.50 0.00 0.00 0.30 87 ∘ Example 2-19 6.00 3.30 0.550.00 0.00 1.00 0.00 0.30 82 ∘ Example 2-20 6.00 6.00 1.00 0.00 0.00 1.000.00 0.30 84 ∘ Example 2-21 7.00 4.20 0.60 0.00 0.00 1.00 0.00 0.30 86 ∘Example 2-22 9.00 6.50 0.72 0.00 0.00 1.00 0.00 0.30 85 ∘ Example 2-2312.00 6.60 0.55 0.00 0.00 1.00 0.00 0.30 83 ∘ Example 2-24 12.00 12.001.00 0.00 0.00 1.00 0.00 0.30 85 ∘ Example 2-25 6.00 3.30 0.55 0.00 0.000.00 2.00 0.30 82 ∘ Example 2-26 6.00 6.00 1.00 0.00 0.00 0.00 2.00 0.3085 ∘ Example 2-27 7.00 4.20 0.60 0.00 0.00 0.00 2.00 0.30 87 ∘ Example2-28 9.00 6.50 0.72 0.00 0.00 0.00 2.00 0.30 86 ∘ Example 2-29 12.006.60 0.55 0.00 0.00 0.00 2.00 0.30 81 ∘ Example 2-30 12.00 12.00 1.000.00 0.00 0.00 2.00 0.30 86 ∘ Example 2-31 6.00 3.30 0.55 0.00 0.00 0.000.00 0.20 81 ∘ Example 2-32 6.00 6.00 1.00 0.00 0.00 0.00 0.00 0.20 84 ∘Example 2-33 7.00 4.20 0.60 0.00 0.00 0.00 0.00 0.20 86 ∘ Example 2-349.00 6.50 0.72 0.00 0.00 0.00 0.00 0.20 85 ∘ Example 2-35 12.00 6.600.55 0.00 0.00 0.00 0.00 0.20 82 ∘ Example 2-36 12.00 12.00 1.00 0.000.00 0.00 0.00 0.20 85 ∘ Example 2-37 6.00 3.30 0.55 0.00 0.00 0.00 0.001.00 81 ∘ Example 2-38 6.00 6.00 1.00 0.00 0.00 0.00 0.00 1.00 85 ∘Example 2-39 7.00 4.20 0.60 0.00 0.00 0.00 0.00 1.00 86 ∘ Example 2-409.00 6.50 0.72 0.00 0.00 0.00 0.00 1.00 85 ∘ Example 2-41 12.00 6.600.55 0.00 0.00 0.00 0.00 1.00 83 ∘ Example 2-42 12.00 12.00 1.00 0.000.00 0.00 0.00 1.00 85 ∘ Example 2-43 6.00 3.30 0.55 0.50 1.00 0.00 1.000.50 84 ∘ Example 2-44 6.00 6.00 1.00 0.50 0.00 0.50 1.00 0.50 87 ∘Example 2-45 7.00 4.20 0.60 1.00 1.00 0.00 1.00 0.50 92 ∘ Example 2-469.00 6.50 0.72 1.00 0.00 0.50 1.00 0.50 90 ∘ Example 2-47 12.00 6.600.55 1.50 1.00 0.00 1.00 0.50 81 ∘ Example 2-48 12.00 12.00 1.00 1.500.00 0.50 1.00 0.50 92 ∘ Comparative 7.00 3.00 0.43 0.50 0.00 0.00 0.000.30 79 x Example 2-1 Comparative 7.00 3.00 0.43 1.50 0.00 0.00 0.000.30 78 x Example 2-2 Comparative 7.00 3.00 0.43 0.00 2.50 0.00 0.000.30 72 x Example 2-3

According to the examples and the comparative example of the embodiment2, a misch metal is used as a rare earth element. The misch metalcontains 50% of cerium, 27% of lanthanum, 11% of neodymium, 5% ofpraseodymium and other rare earth elements for balance, adding up to the100% of misch metal. The major constituents such as cerium, lanthanum,neodymium, and praseodymium occupy 93% from the 100% of misch metal usedin the examples and the comparative example of the embodiment 2.

According to the embodiment 2, the analytical values of the amounts ofcerium, lanthanum, neodymium, and praseodymium are obtained from eachmagnesium alloy and the total amount (%) of cerium, lanthanum,neodymium, and praseodymium is obtained by adding the analytical values.The amount of the misch metal (Mm) is obtained by multiplying the totalamount (%) of cerium, lanthanum, neodymium, and praseodymium by 100/93.Then, the amount of the misch metal is shown in column Mm in Table 2.Accordingly, the amount of the misch metal (Mm) shown in Table 2corresponds the amount of the misch metal containing not only cerium,lanthanum, neodymium, and praseodymium but also other rare earthelements.

As a characteristic evaluation of the series of the embodiment 1, a βphase ratio (area ratio), a bolt-load retention rate (the ratio of thetest piece to ADC12), and the castability (cast cracks) are measured. Asa characteristic evaluation of the series of the embodiment 2, thebolt-load retention rate (the ratio of the test piece to ADC12) ismeasured. The measurement results are shown with the compositions inTables 1 and 2.

In order to measure the β phase ratio, a test piece cutting from acasting is polished and etched with 10 weight % aqueous solution ofacetate for observation. The test piece is observed by a scanningelectron microscopy (SEM) to classify compounds. Additionally, theanalysis is conducted by EDAX to check the presence of the β phase. Inthis case, SEM photographs are taken and the area ratio of the β phaseis obtained by means of an image analysis software (ImagePro and thelikes) to determine the β phase ratio. An average value is taken from 5visual fields to be used as the area ratio.

In the axial force test, a u-shaped die-casting 150 is formed. Asillustrated in FIG. 2, the die casting 150 has arms 151 and 152 whichare formed by a magnesium alloy. The die casting 150 is cast in thefollowing condition. The injection rate (plunger moving speed) is0.3˜0.35 meter/second, the injection pressure is 28 Mpa, injectionmolten metal temperature is liquidus temperature +30 degrees C.,pressing time is 5 seconds, and mold temperature is room temperature ofup to 40 degrees C. A ring test piece 100 is cut from one of the arms151 of the die casting 150 and the test piece 100 is used as a fasteningportion (outer diameter 20 mm, inner diameter (bolt through hole) 9 mm,thickness approximately 10 mm). As schematically illustrated in FIG. 3,a bolt 200 having an external thread is inserted through a bolt throughhole of the test piece 100 via a washer 105 (outer diameter 18 mm,thickness 3 mm, A6061-T6) and is tightened to a screw hole 301 of acounter member 300. The bolt 200 is made of steel, M8×25, strength grade10.9. The counter member 300 is an aluminum die casting alloy, ADC12 inJIS standard (Japanese Industrial Standards). The bolt 200 is tightenedwith 8 kN of an initial axial force. The axial force is measured byusing a strain gauge 400 attached to the bolt 200. Subsequently, a testpiece consisting of the test piece 100 and the counter member 300tightened to the test piece 100 by the bolt 200 is inserted into an airatmosphere furnace to be held at a high temperature (175 degrees C.) for300 hours and then be cooled down to room temperature. After theprocess, the axial force is re-measured and the bolt-load retention rateagainst the initial axial force is obtained. In this case, the averagevalue of plural test pieces is used as the bolt-load retention rate. 76%of the bolt-load retention rate means that the axial force of the testpiece held at a high temperature in the above condition is reduced tothe axial force 8 kN (the initial axial force)×0.76. The axial force ofthe bolt 200 is also measured by an ultrasonic axial force measurementmethod and the similar results to that of the strain gauge measurementare obtained. As the bolt-load retention rate, the ratio of thebolt-load retention rate of each alloy to the bolt-load retention rateof an ADC12 alloy is obtained, provided that the bolt-load retentionrate of the ADC12 alloy is 100. ◯ is filled in evaluation column if thealloy has the bolt-load retention rate exceeding 80 and × is filled inthe evaluation column if the bolt-load retention rate does not exceed80.

Further, a die casting 302 is produced in a shaped illustrated in FIG. 4as a sample, and examined the die casting 302 for the occurrence ofcrack by the naked eye. The casting condition is as follows. Theinjection rate (plunger moving speed) is 1 meter/second, the injectionpressure is 64 MPa, temperature of the mold is 200 degrees C., andmolten metal temperature is liquidus temperature+30 degrees C.

In comparative examples shown in Table 1, one of the amount of aluminumand the Ca/Al ratio is not in the specified range of the examples of theembodiment 1. As shown in Table 1, the β phase ratio is high and thebolt-load retention rate is low in many comparative examples shown inTable 1 because either the amount of aluminum or the Ca/Al ratio is notproper in those comparative examples. Consequently, overall judgmentsfor the comparative examples are × (unsatisfactory). The bolt-loadretention rate is high in comparative examples 1-7 and 1-8, however, thecasting cracks occur and the overall judgment is × (unsatisfactory).

Compared to the comparative examples, the amount of aluminum, the amountof calcium, and the Ca/Al ratio are properly set for the series of theexamples of the embodiment 1. As shown in Table 1, the magnesium alloyscomprehensively excels in terms of the β phase ratio, the bolt-loadretention rate, and the prevention of the casting cracks. The reason forthe excellence is inferred that the occurrence of the β phase isrestrained and boundary sliding is effectively prevented in the hightemperature range.

The bolt-load retention rate is obtained for the series of the examplesof the embodiment 2 as well as the series of the examples of theembodiment 1. ◯ is filled in evaluation column if the alloy has thebolt-load retention rate exceeding 80 and × is filled in the evaluationcolumn if the bolt-load retention rate does not exceed 80. The amount ofaluminum and the Ca/Al ratio is properly set for the series of theexamples of the embodiment 2. As shown in Table 2, the magnesium alloysfor the series of the examples of the embodiment 2 are excellent in theβ phase ratio and the bolt-load retention rate. In addition, the alloysare excellent in the prevention of the cast cracks. As shown in Table 2,the bolt-load retention rate is low and the overall judgment is ×(unsatisfactory) for comparative examples 2-1, 2-1 and 2-3.

Furthermore, a creep resistance test at a range of high temperature isconducted. The test is conducted under the following condition. Themeasured temperature is 180 degrees C., the initial stress is 104 Mpa,the shape of the test piece is a cylinder rod (parallel portion: sixmillimeter diameter), and the measurement time is 300 hours. The resultsof the minimum strain rate are shown in FIG. 5. As shown in FIG. 5, thestrain rate is considerably large in a comparative example 3(Ca/Al=0.43, Mg-7% Al-3% Ca-0.3% Mn). On the other hand, the strain rateis excellent in the order of an example 3-1 (Ca/Al=0.71, Mg-7% Al-5%Ca-0.3% Mn), an example 3-2 (Ca/Al=0.71, Mg-7% Al-5% Ca-0.3% Mn-0.5%Sr), and an example 3-3 (Ca/Al=0.75, Mg-12% Al-9% Ca-0.3% Mn-0.5% Sr).Comparison of the amount of Sr between the example 3-1 and the example3-2 shows that addition of Sr is effective for reduction of the strainrate in the creep resistance test.

(Metal Structure)

In FIGS. 6˜13, photographs (SEM) of metal structures are shown. In FIGS.10 and 13, the photographs of the metal structures of the examples ofthe embodiment 1 are shown. In the photographs, the portions indicatedby black triangles exhibit the β phase. The metal structures areobserved after being etched with 10 weight % aqueous solution ofacetate. As it is understood from the photographs, the β phase is formedin the grain boundary for the alloys of the comparative examples of theembodiment 1. In some cases, the β phase is formed in crystal grains. Onthe other hand, the formation of the β phase is prevented by thecompositions used for the examples of the embodiment 1 and the β phaseis practically 0%. It is inferred that the enhancement of the heatresistance (the creep resistance and the likes) of the magnesium alloyis possible because the formation of the β phase is restrained toeffectively prevent boundary sliding. The β phase is identified by adevice (SEM-EDX) having an electron scanning microscope function and anenergy dispersive X-ray analysis function.

The embodiments of the invention are not to be considered limited towhat are shown in the drawings and described above. It is possible tomake appropriate modifications as necessary. For example, it may use oneor more than one of scandium, gadolinium, terbium, samarium, holmium,thulium, erbium, europium, and ytterbium as rare earth elements as wellas cerium, lantern, neodymium, and praseodymium. The amounts of eachalloying element described in Table 1 can be stated as upper limits andlower limits which define the compositions described in each claim.

INDUSTRIAL APPLICABILITY

The invention can be used for vehicles and components of industrialmachineries, which are expected to reduce weight. In the vehicles, theinvention is used for engine components, for example, an oil pan, atransmission case, a cylinder block, a cylinder head, a piston or thecomponents which requires both lightweight properties and the heatresistance.

The inventors of the invention work hard on developments ofmagnesium-aluminum-calcium based alloys. Observing a composition of amagnesium-aluminum-calcium based alloy, generally, the alloy is likelyto contain two or three phases among the Mg phase (including Mg—Al solidsolution and Mg—Ca solid solution), the β phase (Mg₁₇Al₁₂), the Al₂Ca(Mg) phase, and the Mg₂Ca (Al) phase as main structures.

In a relatively low temperature range, in other words, in a temperaturerange that is equal to or less than 120 degrees C., when the β phase,the Al₂Ca (Mg) phase, and the Mg₂Ca (Al) exist in a grain boundary ofthe Mg phase, boundary sliding is restricted to improve the creepresistance more easily.

However, according to the research findings of the inventors of thisinvention, it has been observed that the β phase is likely to preventthe improvements of creep resistance properties in a high temperaturerange (a temperature range which is over 120 degrees C.). Also, it hasbeen observed that the Al₂Ca (Mg) phase is effective for the improvementof the creep resistance properties.

The inventors have developed the study based on the above concept. As aresult, it has been observed that if the calcium/magnesium mass ratio isset in a range 0.55˜1.0 in the magnesium alloy, which contains aluminum,calcium, and manganese, more specifically, contains aluminum of 6˜12%and manganese of 0.1˜1.5% by weight, a structure where the formation ofthe β phase is restrained with the Mg phase and the Al₂Ca (Mg) phasebeing the fundamental structures (a small amount of the Mg₂Ca(Al) phaseis contained in some cases) is formed to obtain a magnesium alloy whichexcels in the further heat resistance (for example, the creepresistance) and the castability. The observation is confirmed by testsand then the magnesium alloy according to the invention is completed.

Namely, the magnesium alloy, according to the invention, containscalcium, and manganese and is characterized in that aluminum of 6˜12%and manganese of 0.1˜1.5% by weight is contained, the calcium/magnesiummass ratio is in a range of 0.55˜1.0 and the balance is magnesium andinevitable impurities.

According to the invention, it is possible to provide a magnesium alloyand which excels in the heat resistance and the castability.

The principles, of the preferred embodiments and mode of operation ofthe present invention have been described in the foregoingspecification. However, the invention, which is intended to beprotected, is not to be construed as limited to the particularembodiment disclosed. Further, the embodiment described herein are to beregarded as illustrative rather than restrictive. Variations and changesmay be made by others, and equivalents employed, without departing fromthe spirit of the present invention. Accordingly, it is expresslyintended that all such variations, changes and equivalents that fallwithin the spirit and scope of the present invention as defined in theclaims, be embraced thereby.

1. A magnesium alloy containing aluminum, manganese and calcium,comprising: 6˜12% by weight of aluminum; 0.1˜1.5% by weight ofmanganese; a calcium/aluminum mass ratio being 0.55˜1.0; and balancebeing magnesium and inevitable impurities.
 2. A magnesium alloyaccording to claim 1, further comprising; at least one of equal to orless than 1.5% by weight of strontium, equal to or less than 2.5% byweight of rare earth element, equal to or less than 1% by weight ofsilicon; and equal to or less than 2% by weight of tin.
 3. A magnesiumalloy according to claim 1, wherein the calcium/aluminum mass ratio is0.60˜0.88.
 4. A magnesium alloy according to claim 1, wherein themanganese is equal to or less than 0.2˜1.0% by weight.
 5. A magnesiumalloy according to claim 1, wherein the calcium is equal to or greaterthan 4% by weight.
 6. A magnesium alloy according to claim 1, whereinthe calcium is equal to or greater than 5% by weight.
 7. A magnesiumalloy according to claim 1, wherein the magnesium alloy includes a phase(Mg₁₇Al₁₂) and the β phase (Mg₁₇Al₁₂) is equal to or less than 0.5% inan area ratio.
 8. A casting characterized in being formed from amagnesium alloy according to claim 1.